Haruka. Her team is most balanced of all. Even though Iris' Dragonite would give her a lotta troubles, I believe she would win.
Hikari's team also isn't bad at all but... Piplup? No chance. That stubborn penguin still did not evolve so there goes Hikari's chance.
And Kasumi? No point even bothering. She's a Gym Leader so paradoxically she has the least chance as she has monotype team.
Imo: Haruka would win and Iris and Hikari would be battling for the second place.

That's from game point of view.
If we took anime point of view into consideration, anything could happen.